Elisabeth and Mark Horst, artists in Albuquerque who earn $24,000 a year between them, qualified for a zero-premium plan.Credit
Mark Holm for The New York Times

Millions of people could qualify for federal subsidies that will pay the entire monthly cost of some health care plans being offered in the online marketplaces set up under President Obama’s health care law, a surprising figure that has not garnered much attention, in part because the zero-premium plans come with serious trade-offs.

Three independent estimates by Wall Street analysts and a consulting firm say up to seven million people could qualify for the plans, but federal officials and insurers are reluctant to push them too hard because they are concerned about encouraging people to sign up for something that might ultimately not fit their needs.

The bulk of these plans are so-called bronze policies, the least expensive available. They require people to pay the most in out-of-pocket costs, for doctor visits and other benefits like hospital stays.

Supporters of the Affordable Care Act say that the availability of free-premium plans — as well as inexpensive policies that cover more — shows that it is achieving its goal of making health insurance widely available. A large number of those who qualify have incomes that fall just above the threshold for Medicaid, the government program for the poor, according to an analysis by the consulting firm McKinsey and Company.

“The whole point of the law was not only to cover the uninsured, but so people didn’t have to make choices between food or drugs, or going to the doctor or dentist,” said Karen Davis, a health policy expert at the Johns Hopkins Bloomberg School of Public Health. “It’s what it is designed to do.”

Many insurers tried to price their least expensive plans so they would become free or nearly free with the addition of subsidies that are set based on a person’s income and the cost of a midlevel, or silver, plan.

Independence Blue Cross in Philadelphia has four plans that are free to some customers. But the company, along with other insurers, has been careful not to publicize its free coverage for fear of alienating customers who will need to pay more.

“We’re not advertising zero dollar,” said Brian Lobley, a senior vice president at Independence Blue Cross. But the company is promoting monthly premiums in the $20 to $30 range, he said.

The Obama administration has also stressed affordability over coverage with no monthly charge, frequently saying that the cost of coverage will be less than a monthly cellphone bill for many consumers. Officials at the Department of Health and Human Services would not comment on the McKinsey analysis, saying in a statement that the goal of the health law was to provide a range of options for people with differing needs and budgets.

The analysis found that five million to six million people who are uninsured will qualify for subsidies that will be greater than the cost of the cheapest bronze or silver plan. A million more people with individual insurance could also be eligible, according to McKinsey, although estimates of the size of the market for private individual insurance vary widely. None of the people in the analysis qualify for Medicaid.

The availability of zero-premium plans may make the deal especially enticing to the healthy young people the marketplace needs to succeed, said Mark V. Pauly, a professor of health care management at the University of Pennsylvania’s Wharton School. “This is such a good deal that you’d have to believe you were immortal not to really pick it up,” he said.

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Although they vary in their design, bronze plans generally cover about 60 percent of a person’s medical costs. All plans, including bronze, must cover standard benefits like prescription drugs, maternity care and mental health treatment.

The availability of the zero-premium plans varies across the country. McKinsey found that about 40 percent of the uninsured in Missouri will be able to select a no-cost bronze plan, for example, compared with 2 percent of the uninsured in New Jersey.

Its estimate, based on an analysis of premiums for plans offered in the marketplaces in all 50 states and the District of Columbia, is in line with two other estimates, by Credit Suisse and Morgan Stanley.

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The McKinsey researchers also found that about half of the people eligible for zero-premium plans were under 39 and uninsured. The Obama administration has been emphasizing the affordability of its plans for young people, a critical group because their participation in the marketplaces will help keep overall premiums low.

It is impossible to know who will actually sign up, and whether they will choose a zero-premium plan.

For many people, paying slightly more for a silver plan may be a much better option, experts said. Ninety percent of those who will have the option of buying the no-cost plans make less than 250 percent of the federal poverty level, which is $28,725 for an individual, and $58,875 for a family of four. People earning below those thresholds are eligible for the most generous assistance, but only if they choose a silver plan.

About a million of those who will qualify for free coverage will be able to buy a silver plan for no monthly cost. McKinsey, which is releasing a report about the new insurance marketplaces, estimates that the cost of silver plans for the people who qualify for a zero-premium bronze plan will range from $40 to $50 a month.

“They may be getting zero premiums, but they’re also leaving a lot of money on the table if they don’t enroll in a silver-level plan,” said Sabrina Corlette, a professor at Georgetown University’s Health Policy Institute.

All plans, including bronze policies, limit annual out-of-pocket costs to $6,350 for individuals and $12,700 for families. But insurers and advocates said out-of-pocket costs — even those under that limit — can be daunting to people with low incomes.

For Mark and Elisabeth Horst, both artists in Albuquerque, the risks of signing up for a bronze plan were outweighed by the prospect of getting it free. The Horsts, who make $24,000 a year between them, qualified for $612 in monthly subsidies, but the cost of a bronze plan was $581 a month.

“We’re in good health,” Mr. Horst said.

Besides, he said, they can always switch to a better plan next year. “At this point, it’s a little bit of a gamble.”

Not everyone selects the cheapest option. Dante Olivia Smith, a lighting designer from Manhattan, learned that federal subsidies would allow her to buy a bronze plan for $24 a month.

“It was astounding,” she said. “I almost started crying, and called my mom.”

In the end, however, she went with a silver plan for $91 a month that included dental and vision coverage. Ms. Smith, who is 30, said she opted for the more comprehensive plan because of her work, which requires her to climb ladders and use power tools.

“If I had a different job, for 24 dollars a month I would have been like ‘Woo-hoo!' ” she said. “But the reality is, I know what my risks are in my life.”

A version of this article appears in print on November 4, 2013, on Page A1 of the New York edition with the headline: Millions Eligible For Free Policies Under Care Act. Order Reprints|Today's Paper|Subscribe